hur in
carbon bisulphide, are heated in a test-tube placed in a water-bath
until effervescence ceases, then in boiling brine for one hour or longer
when only small quantities are present. The presence of cotton-seed oil
is denoted by a pink coloration. The reaction is rendered much more
rapid, according to Rupp (_Z. Untersuch. Nahr. Genussm._, 1907, 13, 74),
by heating in a stoppered flask.

Other bodies which it is occasionally necessary to test for or determine
in soap include:--

_Carbolic acid._--Fifty grammes of the soap are dissolved in water and
20 c.c. of 10 per cent. caustic potash added. The solution is treated
with an excess of brine, the supernatant liquor separated, and the
precipitate washed with brine, the washings being added to the liquor
withdrawn. This is then evaporated to a small bulk, placed in a Muter's
graduated tube, and acidified with mineral acid.

The volume of separated phenols is observed and stated in percentage on
the soap taken.

Or the alkaline layer may be rendered acid and steam distilled; the
distillate is made up to a known volume, and a portion titrated by the
Koppeschaar method with standard bromine water.

_Glycerine._--Five grammes of soap are dissolved in water, decomposed
with dilute sulphuric acid, and the clear fatty acids filtered and
washed. The filtrate is neutralised with barium carbonate, evaporated
to 50 c.c., and the glycerol estimated by the bichromate method detailed
under Crude Glycerine.

_Starch_ or _gum_ may be detected by dissolving the soap in alcohol,
filtering, and examining the residue on the filter paper. Starch is
readily recognised by the blue coloration it gives with a solution of
iodine in potassium iodide.

_Sugars_ are tested for by means of Fehlings' solution, in the liquor
separated from the fatty acids, after first boiling with dilute acid to
invert any cane sugar.

_Mercury_ will be revealed by a black precipitate produced when
sulphuretted hydrogen is added to the liquor separated from the fatty
acids, and may be estimated by filtering off this precipitate on a tared
Gooch's crucible, which is then dried and weighed.

_Borax or borates_ are tested for in the residue insoluble in alcohol.
This is dissolved in water, rendered faintly acid with dilute
hydrochloric acid, and a strip of turmeric paper immersed for a few
minutes in the liquid. This is then dried in the water-oven, when if any
boric acid compound is present, a bright reddis